1. Field of the Invention
The present invention relates to a zoom lens system for a compact camera, and in particular, relates to a focusing method of the zoom lens system.
2. Description of the Related Art
Unlike a zoom lens system for a single lens reflex (SLR) camera, a zoom lens system for a compact camera does not require a longer back focal distance behind the photographing lens system. Accordingly, in a compact camera, a telephoto type zoom lens system has been employed, while in a SLR camera, a retrofocus zoom lens system has been employed for the purpose of securing a back focal distance.
In such a zoom lens system of a compact camera, a high zoom ratio has been required in recent years, i.e., the shorter focal length at the short focal length extremity, and the longer focal length at the long focal length extremity. Accordingly, a three-lens-group zoom lens system is frequently employed. In this three-lens-group zoom lens system, zooming is performed by moving each of three lens groups independently along the optical axis; and focusing is performed by moving at least one lens group, which usually is the first lens group or the second lens group, along the optical axis. In order to achieve miniaturization of a three-lens-group zoom lens system of this type, it is essential to decrease the overall length of the zoom lens system, the diameter and thickness of each lens group. However, if the number of lens elements is reduced for the purpose of reducing the overall thickness of each lens group, the number of lens elements of a lens group for focusing (hereinafter, focusing lens group) is also reduced. As a result, with respect to any focal length in the entire zooming range defined by the short focal length extremity and the long focal length extremity, it becomes difficult to adequately correct aberrations at focal lengths, with respect to an infinite photographing distance over the closest photographing distance. This tendency becomes more noticeable when a zoom ratio is set higher. Therefore in order to correct aberrations adequately over the entire photographing range (an infinite photographing distance to the closest photographing distance), the number of lens elements has to be increased. Consequently, miniaturization of the three-lens-group zoom lens system cannot be attained.
It is an object of the present invention to provide a miniaturized telephoto-type three-lens-group zoom lens system which is constituted by a small number of lens elements, and by which aberrations can be adequately corrected.
The present invention is based on conception that the number of lens elements of the focusing lens group can substantially be increased if the first lens group and the second lens group are integrally moved upon focusing. According to this arrangement, the correcting of aberrations over the entire photographing range becomes easier. This conception is materialized by utilizing a zoom lens system including a positive first lens group, a positive second lens group, and a negative third lens group, in this order from the object. Upon zooming from the short focal length extremity toward the long focal length extremity, the three lens groups are moved independently along the optical axis so that the distance between the first lens group and the second lens group increases, and the distance between the second lens group and the third lens group decreases.
In other words, under the above-explained zoom lens system, the first lens group and the second lens group which are independently moved upon zooming are arranged to move integrally upon focusing.
The zoom lens system preferably satisfies the following condition:
10 less than KF less than 25xe2x80x83xe2x80x83(1)
wherein
KF=(fT/fFT)2;
fT designates the focal length of the entire zoom lens system at the long focal length extremity, and
fFT designates the combined focal length of the first lens group and the second lens group at the long focal length extremity.
The zoom lens system can satisfy the following condition:
0.1 less than (d12Txe2x88x92d12W)/fW less than 0.3xe2x80x83xe2x80x83(2)
wherein
d12T designates the distance between the most image-side surface of the first lens group and the most object-side surface of the second lens group, at the long focal length extremity;
d12W designates the distance between the most image-side surface of the first lens group and the most object-side surface of the second lens group, at the short focal length extremity; and
fW designates the focal length of the entire zoom lens system at-the short focal length extremity.
For example, if an attempt is made to constitute the first lens group by three lens elements, and the second lens group by two lens elements, the correcting of aberrations over the entire photographing range can relatively be made adequately.
According to another aspect of the present invention, there is provided a focusing method for the zoom lens system including a positive first lens group, a positive second lens group, and a negative third lens group, in this order from the object; and the method including the following steps:
upon zooming from the short focal length extremity toward the long focal length extremity, the three lens groups are moved independently along the optical axis so that the distance between the first lens group and the second lens group increases, and the distance between the second lens group and the third lens group decreases; and
upon focusing, the first lens group and the second lens group are integrally moved.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-189884 (filed on Jun. 23, 2000) which is expressly incorporated herein by reference in its entirety.